1. Field of the Invention
This invention relates to a multi-wavelength optical reflector which includes a diffraction grating structure and a method of making such a reflector. More especially, although not exclusively, the invention relates to an optical reflector suitable for use with, or as part of, a wavelength division multiplexed (WDM) optical communications system.
2. Description of the Related Art
Narrowband optical reflectors/filters are important for a number of applications in optical telecommunications and signal processing including multiple channel optical telecommunications networks using wavelength division multiplexing (WDM). Such networks can provide advanced features, such as wavelength routing, wavelength switching and wavelength conversion, adding and dropping of channels and wavelength manipulation in much the same way as time slot manipulation in time division multiplexed systems.
WDM is rapidly emerging as a key technology for optical networking, but implementing it requires the development and optimization of many of the optical devices required within such systems. For example, tuneable optical sources are required which can provide the many different wavelengths required for the many different channels. In addition, filters such as comb filters and passband filters are required to exclude spurious signals and wavelength monitoring is required to avoid straying off channel and contaminating the network. Generally such networks must include optical amplifiers, such as the erbium doped fibre amplifier, which currently limits the overall bandwidth to approximately 35 nm. As a consequence, much work is tending to concentrate on developing optical components which operate within the erbium bandwidth window.
A laser suitable for an optical telecommunications network is a Distributed Feedback (DFB) laser diode. Its spectral properties depend principally on the presence of a diffraction grating structure which can be in the form of a surface-relief structure or buried within the device. The grating can be formed as a corrugation which acts as a periodically varying optical waveguide boundary and allows coupling between the forward and backward propagating waves. A DFB laser diode incorporating such a grating structure operates in a first-older spatial harmonic mode whether directly in the first-order or indirectly at the first-order spatial harmonic component of a multiple order grating.
It is common for DFB lasers to include end reflectors, such that the longitudinal mode spectrum is asymmetrically distributed about the Bragg wavelength. This type of DFB laser has one dominant mode and therefore has a stable output at a given wavelength.
U.S. Pat. No. 4,896,325 proposes a wavelength tuneable laser having sampled gratings at the front and rear of its gain region. The gratings produce slightly different reflection combs which provide feedback into the device. The gratings can be current tuned in wavelength with respect to each other. Coincidence of a maximum from each of the front and rear gratings is referred to as supermode. To switch the device between supermodes requires a small electrical current into one of the gratings to cause a different pair of maxima to coincide in the manner of a vernier. By applying different electrical currents to the two gratings, continuous tuning within a supermode can be achieved. For optimum operation of such a laser, the diffraction gratings should ideally have a flat reflection spectrum, that is, it should comprise a plurality of reflection maxima of equal intensity. In practice however, the reflection spectra of the known sampled grating structures have a Gaussian type envelope which limits the total optical bandwidth over which the laser can reliably operate as a single mode device. Additionally, as a consequence of the non-uniform reflection spectra of the gratings, the output power of the laser as a function of wavelength is not uniform. Therefore a need exists for a grating structure having an improved reflection spectrum.
U.S. Pat. No. 5,325,392 discloses a distributed reflector structure which comprises a diffraction grating having a repeating unit of constant length which defines a modulation period and at least one of a number of parameters that determine an optical reflectivity of the grating and varies in dependence on its position in each of the repeating units. In one embodiment, the parameters comprises changing (shortening or lengthening) the pitch of the grating within a single grating period at the selected positions in a repeating unit. These changes in pitch are equivalent to a progressive change in the phase and the resulting structure is thus a chirped grating structure. Since the device requires a number of gradual and progressive changes in pitch, the only techniques available to make the device are electron beam techniques which are expensive and do not lead themselves to large scale production.
The present invention has arisen in an endeavour to provide a multi-wavelength comb reflector filter for use in a WDM system which in part at least overcomes the limitations of the known reflectors.
According to the present invention, a multi-wavelength optical reflector comprises: a diffraction grating structure comprising a plurality of repeat grating units in which each grating unit comprises a series of adjacent diffraction gratings having the same pitch, wherein grating units and adjacent gratings within a grating unit are separated by a phase change of substantially pi (xcfx80) radians and wherein at least two of the gratings within a grating unit have different lengths, the lengths being selected so as to provide a predetermined reflection spectrum. Since the phase changes within a grating unit are substantially pi radians, this enables the device to be readily manufactured using holographic and photolithographic techniques.
A predetermined reflection spectrum is one in which the reflectivity of individual reflection maxima are chosen to provide an optimum response from the device for its particular application. Typically a predetermined reflection spectrum has a number of individual reflection maxima which have substantially equal reflectivities.
Advantageously adjacent grating units and/or adjacent gratings within a grating unit are substantially contiguous. Although in practice, small gaps between adjacent grating units and/or adjacent gratings may be present when using phase mask holographic techniques to define the phase shifts, such gaps are not found to affect the reflection spectrum appreciably.
Conveniently the lengths of the gratings are selected by superposing a plurality of periodic variations having different periods to form a periodic waveform having a multi-lobed envelope and the relative lengths of the gratings are selected to correspond with the relative lengths at which the envelope tends to zero.
With such a selection method, phase shifts are preferably imparted to individual ones of the plurality of periodic variations. Preferably pairs of the phase shifts are imparted to individual ones of the plurality of variations, the pairs of phase shifts having equal magnitude but opposite signs. A grating structure constructed having grating lengths selected in his manner is found to have more uniform reflection spectrum.
Advantageously the periodic variations are not all in phase at any point along the length over which they are superposed. It will be appreciated that the described selection method can alternatively be used to determined an initial position for the phase shifts and the phase shifts optimised using an iterative process to obtain the desired reflection spectrum.
In a preferred implementation the reflector comprises a waveguide device, though it can alternatively be fabricated as a transmission grating or could, for example, be incorporated as part of a Fabry Perxc3x3t cavity. The reflector of the present invention can be incorporated into a filter or a laser or can be incorporated as part of a wavelength division multiplexer or demultiplexer.
According to yet a further aspect of the invention, there is provided a method of making a multi-wavelength optical reflector as described above, characterized in that a single exposure step is required to produce a pattern from which the diffraction grating structure can be obtained. In using a single exposure step, the problems encountered with transferring between sequential exposure steps are avoided.